Transport Mechanisms Flashcards
What is migration?
The transport of charged species due to electric fields, from high to low potentials
What are the 3 transport mechanisms in an electrochemical cell?
Migration, diffusion and convection
What are the 3 main driving forces, transport processes and coupling coefficients associated with charge transport?
1) Electrical driving force = Conduction - electrical potential gradient - conductivity
2) Chemical driving force = Diffusion - concentration gradient - concentration
3) Mechanical driving force = Convection - pressure gradient - viscosity
What is the term used for voltage loss due to intrinsic resistance in conductors?
Ohmic loss
Define ‘conductivity’
The ability of a material to permit the flow of charge when driven by an electric field
What are the 2 main factors influencing a material’s conductivity?
1) How many carriers are available
2) The mobility of carriers within the material
Why is ionic conduction more difficult than electronic conduction?
Because ions are larger and have limited pathways due to a number of available vacancies and interstitials
True or false: Ion mobility is limited by the rate at which ions can ‘hop’ across the crystal lattice (via vacancies and interstitials)
True
How is the effectiveness of ion ‘hopping’ characterised?
By the material’s diffusivity
What law governs diffusion due to concentration gradient?
Fick’s law
What is the molar flux?
The rate of material that passes through a plane of unit area
What is convection in an electrochemical system?
The bulk movement of the electrolyte and the interactions between ions and solvent (dilute solutions)
What are the units of current density?
A m^-2
Which of the following has the largest concentration overpotential across the membrane?
(a) LIB
(b) Fuel cell
(c) Electrorefining cell (copper)
(b) Fuel cell.
Describe the structure of a SOFC
Membrane - electrolyte (prevents flow of electrons)
Electrode - diffusion
Channel that distributes the gas by convection
